The detection and rapid avoidance of noxious thermal stimuli is crucial for survival (Basbaum et al., 2009). Both painful and innocuous thermal stimuli are conveyed by primary afferent sensory neurons that innervate skin and mouth and have their cell bodies in the trigeminal (TG) and dorsal root ganglia (DRG) (Basbaum et al., 2009; Caterina, 2007). Accumulating evidence indicates that the detection of thermal stimuli in mammals strongly depends on the activation of temperature-sensitive non-selective cation channels of the TRP superfamily (Bandell et al., 2007; Basbaum et al., 2009; Caterina, 2007; Talayera et al., 2008). TRPM8 and TRPA1 were shown to be activated by cooling (McKemy et al., 2002; Peier et al., 2002a; Story et al., 2003) and to mediate cold responses in TG and DRG neurons (Bautista et al., 2007; Colburn et al., 2007; Dhaka et al., 2007; Karashima et al., 2009). Consequently, knockout mice lacking either TRPM8 or TRPA1 exhibit specific behavioral deficits in response to cold stimuli (Bautista et al., 2007; Colburn et al., 2007; Dhaka et al., 2007; Kwan et al., 2006; Nilius and Voets, 2007), although the involvement of TRPA1 in cold sensing in vivo remains a matter of debate (Bautista et al., 2006; Karashima et al., 2009; Knowlton et al., 2010; Kwan et al., 2006). Oppositely, four members of the TRPV subfamily, TRPV1-4, are activated upon heating (Caterina et al., 1999; Caterina et al., 1997; Chung et al., 2003; Guler et al., 2002; Peier et al., 2002b; Smith et al., 2002; Watanabe et al., 2002; Xu et al., 2002). TRPV1, a heat and capsaicin sensor expressed in nociceptor neurons is involved in detecting heat-evoked pain, particularly in inflamed tissue (Caterina et al., 2000; Caterina et al., 1997; Davis et al., 2000; Tominaga et al., 1998). The related TRPV3 and TRPV4 are strongly expressed in skin keratinocytes, and have been mainly implicated in sensing innocuously warm temperatures (Chung et al., 2003; Chung et al., 2004; Lee et al., 2005; Moqrich et al., 2005; Peier et al., 2002b; Smith et al., 2002; Xu et al., 2002). TRPV2 is activated by extreme heat (>50° C.) (Caterina et al., 1999), and has been considered as a potential molecular candidate to explain the activation of TRPV1-deficient sensory neurons at temperatures above ˜50° C., as well as the residual nocifensive response to noxious heat stimuli in TRPV1-deficient mice (Caterina et al., 2000). However, it remains to be established whether TRPV2 functions as a thermosensor in vivo, as deficits in detecting noxious heat have not yet been described for TRPV2-deficient mice. Moreover, it has been clearly demonstrated that a large fraction of heat-sensitive nociceptors lack expression of both TRPV1 and TRPV2 (Woodbury et al., 2004). Thus, the molecular basis of TRPV1-independent noxious heat sensing in the somatosensory system is currently unknown (Basbaum et al., 2009).
TRPM3 is a member of the melastatin subfamily of TRP channels with limited homology to the heat-sensitive TRPV channels. It is expressed in a variety of neuronal and non-neuronal tissue (Grimm et al., 2003; Lee et al., 2003; Oberwinkler and Philipp, 2007). The TRPM3 gene encodes for different TRPM3 isoforms due to alternative splicing and exon usage, leading to channels with divergent pore and gating properties (Oberwinkler et al., 2005). The neurosteroid pregnenolone sulphate (PS) is currently the most potent known activator of TRPM3 (in casu, the (2 isoform, Wagner et al., 2008), and PS-induced activation of TRPM3-like currents has been linked to vascular smooth muscle contraction and Ca2+-induced insulin release from pancreatic islets in vitro (Naylor et al., 2010; Wagner et al., 2008). However, it is currently unclear whether PS-induced gating of TRPM3 is occurring in vivo, and the physiological roles of the channel remain largely unclear (Nilius and Voets, 2008).
Previous studies demonstrating expression of TRPM3-encoding mRNA in sensory neurons (Lechner et al., 2009; Nealen et al., 2003; Staaf et al., 2010), and PS-induced pain responses in mice (Ueda et al., 2001) encouraged us to investigate the possible role of TRPM3 in somatosensation and nociception. In this study we found that TRPM3 is functionally expressed in a large subset of sensory neurons from the dorsal root and trigeminal ganglia (DRG and TG), and accounts for the majority of PS responses in these cells. Intraplantar injection of PS evokes nocifensive responses in wild type mice but not in Trpm3−/− mice, indicating that TRPM3 activation provokes pain. Moreover, we discovered that TRPM3 is activated by heat. Consequently, TRPM3-deficient mice exhibit clear deficits in their avoidance response to noxious heat, but not to noxious cold or mechanical stimuli. TRPM3-deficient mice also failed to develop heat hyperalgesia following an inflammatory challenge. Our results provide evidence that TRPM3 plays a previously unanticipated role in heat sensation and nociception.